The present invention claims the benefit of Korean Patent Application No. 2001-45800, filed in Korea on Jul. 30, 2001, which is hereby incorporated by reference for all purposes as if fully set forth herein.
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an array substrate having color filters for use in the LCD device and a manufacturing method thereof.
2. Discussion of the Related Art
In general, since flat panel display devices are thin, low weight, and have low power consumption, they are increasingly being used for displays of portable devices. Among the various types of flat panel display devices, liquid crystal display (LCD) devices are widely used for laptop computers and desktop monitors because of their superiority in resolution, color image display, and display quality.
LCD devices use the optical anisotropy and polarization properties of liquid crystal molecules to produce a predetermined image. Liquid crystal molecules have a definite orientation that results from their peculiar characteristics. The specific orientation can be modified by an electric field that is applied across the liquid crystal molecules. In other words, electric fields applied across the liquid crystal molecules can change the orientation of the liquid crystal molecules. Due to optical anisotropy, incident light is refracted according to the orientation of the liquid crystal molecules.
Specifically, the LCD devices have upper and lower substrates with electrodes that are spaced apart and face each other, and a liquid crystal material is interposed therebetween. Accordingly, when a voltage is applied to the liquid crystal material by the electrodes of each substrate, an alignment direction of the liquid crystal molecules is changed in accordance with the applied voltage to display images. By controlling the applied voltage, the LCD device provides various transmittances for rays of light to display image data.
The lower substrate, commonly referred to as an array substrate, includes switching elements, such as thin film transistors (TFTs) and pixel electrodes. The thin films for the TFTs on the lower substrate are usually formed by repetitive photolithography. The upper substrate, commonly referred to as a color filter substrate, usually includes a common electrode and color filters. The color filters have red (R), green (G) and blue (B) sub-color filters that are alternately disposed on the upper substrate. The color filters are formed of organic substances and made by a method, such as pigment dispersion, dyeing process or electrostatic painting.
FIG. 1 is a schematic cross-sectional view showing a pixel of a related art liquid crystal display (LCD) panel. As shown in FIG. 1, the related art LCD panel includes a first substrate 11, a second substrate 21 and a liquid crystal layer 30. The upper and lower substrates 11 and 21 are spaced apart from each other, and the liquid crystal layer 30 is interposed therebetween. The upper and lower substrates 11 and 21 are often referred to as an array substrate and a color filter substrate, respectively.
A gate electrode 12 is disposed on a front surface of a first substrate 11, and a gate insulation layer 13 is formed to cover the gate electrode 12. The gate electrode 12 is made of a conductive material, such as metal, and the gate insulation layer 13 is made of a insulating material, such as silicon nitride (SiNx) or silicon oxide (SiO2). An active layer 14 that is made of amorphous silicon is disposed on the gate insulation layer 13, especially over the gate electrode 12. First and second ohmic contact layers 15a and 15b, which are made of doped amorphous silicon, are disposed on the active layer 14. Source and drain electrodes 16a and 16b are formed on the first and second ohmic contact layers 15a and 15b, respectively. The source and drain electrodes 16a and 16b are formed of a conductive material, such as metal. Thus, a thin film transistor (TFT) T on the first substrate 11 includes the gate electrode 12, the active layer 14, the ohmic contact layers 15a and 15b, and the source and drain electrodes 16a and 16b. Although not shown in FIG. 1, the gate electrode 12 is connected to a gate line (not shown), and the data electrode 14 is connected to a data line (not shown). The gate and data lines cross each other so as to define a pixel region. A passivation layer 17 is formed on the gate insulation layer 13 and on the source and drain electrodes 16a and 16b to cover the TFT T. An organic material, such as benzocyclobutene (BCB), or an inorganic material, such as silicon nitride or silicon oxide, is used for the passivation layer 17. The passivation layer 17 has a drain contact hole 17c therethrough to expose a portion of the drain electrode 16b. On the passivation layer 17, a pixel electrode 18 that is made of a transparent conductive material is formed such that the pixel electrode 18 contacts the drain electrode 16b through the drain contact hole 17c. 
Meanwhile, as mentioned before, the second substrate 21 is spaced apart from the first substrate 11 over the TFT T. On the rear surface of the second substrate 21, a black matrix 22 is disposed in a position corresponding to the TFT T of the first substrate 11. Although it is not clearly shown in FIG. 1, the black matrix 22 is actually formed on the whole surface of the second substrate 21 and has an opening that corresponds to the pixel electrode 18 of the first substrate 11. The black matrix 22 prevents light leakage in the LCD panel except for a portion for the pixel electrode 18. The black matrix 22 protects the TFT T from the light such that the black matrix 22 prevents the occurrence of photo current in the TFT T. Color filters 23a and 23b are formed on the rear surface of the second substrate 21 to cover the black matrix 22. Each of the color filters 23a and 23b has one of the red, green and blue colors. The red, green and blue color filters are alternately arranged on the second substrate 21, and each of the red, green and blue color filters corresponds to one pixel region where the pixel electrode 18 is located. A common electrode 24 that is made of a transparent conductive material is disposed on the color filters 23a and 23b all over the second substrate 21. The liquid crystal layer 30 is interposed between the first and second substrates 11 and 21, specifically between the pixel electrode 18 and the common electrode 24.
In the conventional LCD panel mentioned above, the pixel electrode has a one-to-one correspondence with one of the color filters. Namely, after forming the array substrate and the color filter substrate, respectively, the color filter substrate having the color filters is arranged over the array substrate in order to let one pixel electrode correspond to one color filter. However, when arranging the second substrate to the first substrate or vice versa, misalignment can occur between the first substrate and the second substrate, thereby causing malfunction such as light leakage in the LCD panel. To overcome this problem, the black matrix on the second substrate is enlarged. However, in this case of enlarging the black matrix, the aperture ratio of the liquid crystal panel is lessened.
Therefore, the color filters are formed on the array substrate (i.e., the first substrate) to prevent misalignment between the first substrate and the second substrate, thereby reducing the black matrix size and increasing the aperture ratio of the LCD panel. When the color filters are formed between the substrate and the TFT (i.e., beneath the TFT), it is referred to as a Thin film transistor On Color filter (TOC) structure. At this time, only the black matrix and common electrode are formed on the second substrate without the color filters.
As mentioned hereinbefore, the array substrate includes many thin film transistors formed thereon and these thin film transistors are formed by the repetition of deposition and patterning. When patterning the layers and forming the thin film patterns accurately, a plurality of alignment keys should be formed previously at the peripheral portion of the substrate. Since the alignment keys have steps because of their thickness, the alignment keys can be recognized when patterning the opaque layers, such as metal layers. However, in the array substrate having the TOC structure, since the color filters and the overcoat layer on the color filter are relatively thick, the alignment keys are not recognized when patterning the opaque layers, such as metal layers. Therefore, the thin film patterns are not formed properly and misalignment occurs among the thin film patterns.
Accordingly, the present invention is directed to an array substrate for a liquid crystal display (LCD) device, that substantially obviates one or more of problems due to limitations and disadvantages of the related art.
An advantage of the present invention is to provide an array substrate for a liquid crystal display device, which has a high aperture ratio.
Another advantage of the present invention is to provide a method of manufacturing an array substrate for a liquid crystal display device, which promotes accurate alignment when forming thin films.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, an array substrate for use in a liquid crystal display device comprises a transparent substrate that includes a display area and a non-display area; a plurality of red, green and blue color filters that are formed on the transparent substrate in the display area; a first alignment key formed on the transparent substrate in the non-display area and made of the same material as said color filters; an overcoat layer formed on the plurality of red, green and blue color filters to cover said color filters in the display area; gate and data lines formed over the overcoat layer, the gate and data lines crossing each other to define a pixel region in the display area; a thin film transistor formed over the overcoat layer in the display area, the thin film transistor arranged in the pixel region and connected to the gate and data lines; a passivation layer covering the gate and data lines and the thin film transistor in the display area and covering the first alignment key in the non-display area, the passivation layer having a drain contact hole to the thin film transistor; a gate insulation layer interposed between the overcoat layer and the passivation layer in the display area and between the substrate and the passivation layer in the non-display area; and a pixel electrode formed on the passivation layer in the display area, the pixel electrode contacting the thin film transistor through the drain contact hole.
The array substrate for use in the liquid crystal display device further includes a buffer layer between the overcoat layer and the thin film transistor, and a black matrix on the transparent substrate in boundaries of the plurality of red, green and blue color filters. The black matrix is arranged between the transparent substrate and the color filters. The array substrate further includes a second alignment key between the transparent substrate and the first alignment key, wherein the second alignment key is made of the same material as the black matrix. The overcoat layer can cover the first alignment key in the non-display area.
In another aspect, a method of forming an array substrate for use in a liquid crystal display device includes providing a transparent substrate that includes a display area and a non-display area; forming a plurality of red, green and blue color filters on the transparent substrate in the display area; forming a first alignment key on the transparent substrate in the non-display area using the same material as said color filters; forming an overcoat layer on the plurality of red, green and blue color filters to cover said color filters in the display area; forming gate and data lines over the overcoat layer, the gate and data lines crossing each other to define a pixel region in the display area; forming a thin film transistor over the overcoat layer in the display area, the thin film transistor arranged in the pixel region and connected to the gate and data lines; forming a passivation layer to cover the gate and data lines and the thin film transistor in the display area and to cover the first alignment key in the non-display area, the passivation layer having a drain contact hole to the thin film transistor; forming a gate insulation layer between the overcoat layer and the passivation layer in the display area and between the substrate and the passivation layer in the non-display area; and forming a pixel electrode on the passivation layer in the display area, the pixel electrode contacting the thin film transistor through the drain contact hole.
The method further includes forming a black matrix on the transparent substrate in boundaries of the plurality of red, green and blue color filters, wherein the black matrix is arranged between the transparent substrate and the color filters. The method further includes forming a second alignment key between the transparent substrate and the first alignment key, wherein the second alignment key is made of the same material as the black matrix. The overcoat layer can cover the first alignment key in the non-display area.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.